staging: rtl8821ae: add static for local symbols in rtl_btc.c

This commit fixes the sparse warning:

    "warning: symbol 'X' was not declared. Should it be static?"

Where the X is one of the following

    rtl_btc_operation

Signed-off-by: Konrad Zapalowicz <bergo.torino@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>